1. Field of the Invention
This invention relates to a cylinder apparatus for a rack and pinion type power steering mechanism of a motor vehicle or the like, and more particularly it pertains to such a cylinder apparatus which is so designed that steering operation can be performed with accuracy and ease.
2. Description of the Prior Art
There has conventionally been proposed a cylinder apparatus for a power steering mechanism of the aforementioned type, such as shown in FIG. 1. In the conventional apparatus, a gear housing 1 is formed with a hole adapted to constitute a power cylinder 2, and a rack shaft 3 has its rack section 3a inserted in the power cylinder 2 and its fore end 3b projected out of the gear housing 1. A rod bracket (not shown) is attached to the fore end 3b of the rack shaft 3, and two tie rods (not shown) respectively connected to left and right wheels to be steered are tied at one end to the rod bracket.
Furthermore, a single power piston 4 is mounted on the rack shaft 3 at the end of the rack section 3a thereof in such a manner as to be slidable in a guide tube 5 which is fixedly provided in the power cylinder 2. The power piston 4 defines in the power cylinder 2 two pressure chambers 6 and 7 which are filled with operating fluid, which may be pressurized oil, pressurized gas or the like, and communicated with control valve means (not shown) provided in the gear housing 1 through passages 8 and 9 respectively. Coupled to a steering wheel (not shown) is a pinion 10 which is disposed in the gear housing 1 in intermeshing relationship with the rack section 3a of the rack shaft 3 and arranged to actuate the aforementioned control valve means in synchronism therewith. Denoted at 11 is a seal ring.
In the above-mentioned conventional arrangement, when the steering wheel is turned clockwise or anticlockwise, the pinion 10, which is connected to the steering wheel through a steering shaft or the like, will be rotated clockwise or anticlockwise as viewed in FIG. 1 so that the rack shaft 3 having the rack section 3a disposed in intermeshing relationship with the pinion 10 will be urged leftwardly or rightwardly, thus tending to cause the wheels to be steered to the left or right. Thereupon, the control valve means will also be actuated so that high pressure operating fluid available from a source (not shown) will be introduced into the pressure chamber 7 on the right hand side as viewed in FIG. 1 through the passage 9. At the same time, the operating fluid contained in the pressure chamber 6 on the left hand side as viewed in the figure will be exhausted therefrom through the passage 8 in a proportion corresponding to the quantity of operating fluid introduced into the other pressure chamber 7. In this way, an assist force will be produced which will in turn be imparted, together with a force resulting from the clockwise-turning of the steering wheel, to the rack shaft 3, thus assisting the latter in the aforementioned leftward movement. When the steering wheel is turned anticlockwise, the pinion 10 will also be rotated anticlockwise, and thereupon, the control valve means will be actuated reversely so that operating fluid will be introduced into the left hand side pressure chamber 6 while the operating fluid contained in the right hand side pressure chamber 7 will be exhausted therefrom in a proportion corresponding to the quantity of operating fluid introduced into the pressure chamber 6. In this way, an assist force will be produced which will in turn be imparted, together with a force resulting from the anticlockwise-turning of the steering wheel, to the rack shaft 3, thus assisting the latter in the aforementioned rightward movement.
With the foregoing conventional cylinder apparatus, however, since the pressure-applied areas of the power piston 4 are different between the two pressure chambers 6 and 7 due to the construction that the power cylinder 2 is divided into two sections, i.e., the two pressure chambers 6 and 7 by the power piston 4 mounted on the end of the rack shaft 3 and the rack shaft 3 is disposed to extend through the pressure chamber 7 with the fore end 3b thereof being projected out of the power cylinder 2, difficulties are encountered when it is attempted to make the fluid pressures prevailing on the left and right hand sides of the power piston 4 balanced in order to make nil the assist force at the neutral position. Another problem is such that a difference is caused between the steering forces required to steer the wheels to the left and right.